Modulation protective arrangement



1950 J. R. SHERWOOD ETAL 2,534,073

MODULATION PROTECTIVE ARRANGEMENT Filed April 20, 1949 M Q Q Jo/n/ K. SHEA-Mon g (L Kf/Y/YEW L. lumps;

Q INVENTORS.

Patented Dec. 12, 195% UNITED STATES PATENT OFFICE MODULATION PROTECTIVE ARRANGEMENT John R. Sherwood, Cedar Rapids, and Kenneth L. Kllppel, Iowa City, Iowa, assignors to Collins Radio Company, Cedar Rapids, Iowa, a corporation of Iowa Application April 20, 1949, Serial No. 88.644

4 Claims.

This invention relates to wave modulation systems and more especially to protective arrangements for carrier modulation circuits.

In conventional audio frequency modulating systems for radio carriers and the like, it is customary to employ a relatively expensive audiofrequency modulation transformer whose input winding is energized by the audio-frequency signals, and the voltage across the secondary winding determines the modulation of the carrier. It has been found that if, for any reason, the load across the secondary of the modulation transformer is disconnected or open-circuited, excessive voltages are built up which may damage the transformer and its associated equipment.

Accordingly, one ,of the principal objects of this invention is to provide a modulation arrangement for carrier systems wherein the danger of damage to the modulation equipment and especially to the modulation transformer is substantially eliminated.

A feature of the invention relates to a protective circuit arrangement for carrier modulators, wherein the use of electromechanical relays as protective control elements, is avoided.

Another feature relates to a modulation transformer having a special voltage dividing network connected across its secondary winding and including as one of the elements a gas discharge tube, for applying a cutoff negative bias to the grid of the voltage amplifier input tube when the voltage across the secondary of the modulationtransformer tends to rise above a predetermined safe value.

A further feature relates to the novel organization, arrangement and relative interconnection of parts which cooperate to provide a simplified and efficient protective arrangement for carrier modulators, and which can be easily adjusted to correspond with the maximum per cent of modulation desired in the carrier.

Other features and advantages not particularly enumerated, will be apparent after a consideration of the following detailed descriptions and the appended claims.

In the single figure of the drawing, the numeral l represents any well-known source of audiofrequency input signals, such as voice frequency signals, which are applied to the control grid 2 of a speech amplifier tube 3. This tube may have the usual electron-emitting cathode 4, control grid 2, and plate or anode 5, and preferably an additional grid, such as a shield grid. The control grid 2 can be blassed with respect to the cathode 4 in the conventional way by the cathode bias resistor 6 and its by-pass condenser it This bias is chosen so as normally to cause the tube 3 to operate on the linear part of its characteristic curve for the various input signals from source I. However, as will be described hereinbelow, under certain conditions the grid 2 is arranged to have a negative cutoif bias applied to it through resistor 8 and over conductor 4 by the conventional cathode bias resistor l6 and its by-pass condenser ll. A suitable grid leak I 8 provides a direct current return path from the grid l2 to the cathode M. The amplified output of tube I3 is applied to the primary winding I9 of a modulation transformer 20. The secondary winding 2| of this transformer is connected in circuit with the usual direct current plate power supply, represented schematically by the battery 22, and also in circuit with the plate 23 of a grid-controlled modulator tube 24 through a radio frequency choke coil 25.

The high frequency or carrier source 26 is coupled through a tuned input circuit 21 across the control grid 28 and cathode 29 of modulator tube 23, the grid 28 being appropriately negatively biassed with respect to cathode 29 by a suitable voltage represented schematically by battery 30. The tuned output or tank circuit 3| is coupled to the plate 23 through a suitable coupling condenser 32 in the well-known manner. The tube 24 therefore acts as a plate modulator in that its plate voltage is controlled by the voltage across the secondary winding 2| of the modulation transformer which in turn is a function of the audio-frequency modulating signals. It will be understood, of course, that the invention is not limited to a modulator of the plate modulation type, and any other wellknown type of modulation such as grid modulation, constant current modulation and the like, may be employed. The circuit can be adjusted in the well-known manner so that for modulation of the carrier, the peak voltage across the loaded winding 2| is at a known predetermined maximum value. Any voltage in excess of this would cause .damage to the said transformer 20. Such an undesirable increase will occur, for

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example, if the modulator tube load across the secondary winding 2| is removed or open-circuited.

In order to overcome this disadvantage, two resistors 33 and 34 are bridged across winding II with the resistor 34 preferably being adjustable. Connected between the junction point 35 of these two resistors and the conductor 3 is a voltage regulator tube 36, preferably, although not necessarily, of the gaseous discharge type, such for example as a type VR90 voltage regulator tube. The cathode 31 of this tube is connected to the point 35, and the plate or anode 38 is connected through filter resistors 33, 43, to conductor 9. Suitable filter condensers ll and 42 are connected between the resistors 33, 40, and ground.

The resistors 33 and 34 act as a voltage divider. the ratio of the two resistors 33 and 34 being adjusted so that when the peak voltage across winding 2! is below that necessary to achieve 100% modulation, or any other desired maximum percent modulation, in the carrier, insufiicient potential is impressed across cathode 31 and anode 38 to render tube 36 conductive. On the other hand, when the said peak voltage across winding 2| rises to a dangerous value, or above that necessary to achieve the desired per cent modulation of the carrier, sufficient potential is impressed across tube 36 to render it conductive. The tube 36 thereupon acts to produce a rectified negative direct current voltage on conductor 9, the ripples or audio-frequency components be in filtered out by the filter combination 39, 40, ll and 42. The rectified negative voltage on conductor 9 is applied to the control grid 2 through the resistor 8, and is of such a value as to bias the tube 3 to plate current cutoff. This in turn causes a correspondin reduction in the voltage across primary winding 19, and reduces the voltage across winding 2| to the desired safe low value.

With the foregoing arrangement, it will be seen that the excitation of grid 2 is approximately 180 out of phase with the voltages in the winding 21. Consequentl the direct current negative voltage feedback circuit above described does not introduce undesirable regeneration in the system.

One of the important advantages of the system as described, is that the protective feature can be easily adapted to any reasonable level of operation or maximum percentage modulation in the carrier merely by changing the relative values of the two resistors 33 and 34.

While one particular embodiment has been described herein, it will be understood that various changes and modifications may be made thereinwithout departing from the spirit and scope of the invention.

What is claimed is:

l. A carrier modulation system, comprising a source of carrier waves, a source of modulating signals, a modulator tube, a grid-controlled amplifier tube for amplifying said modulating signals, a modulation transformer for coupling the amplified signals to said modulator tube, and means to protect said transformer against excessive voltage rise when the modulator load i removed from the secondary winding of said transformer, the last-mentioned means comprising a iii) voltage divider connected across said secondary winding, a rectifier tube having its anode-cathode discharge path bridged across said voltage divider and normally non-conductive except when the voltage across said secondary winding rises above a predetermined value, and circuit connections from said rectifier tube to said amplifier tube to apply a plate current cut-oi! bias to the grid thereof.

2. A carrier modulation system, comprising a source of carrier waves, a source of modulating signals, a grid-controlled amplifier tube for said signals, a modulation transformer for coupling the amplified signals to the modulator tube, a pair of voltage divider resistors connected across the secondary winding of said transformer, a rectifier tube connected between said voltage divider resistors and the control grid of said amplifier tube, and means to adjust the, relative values of the voltage divider resistors to render said rectifier tube non-conductive except when the voltage developed across said secondary winding rises above a predetermined level, and circuit connections between said rectifier tube and the grid of said amplifier tube for applying a plate current cut-oil bias to said grid when the voltage developed across said secondary winding rises above said predetermined value.

3. A modulation system, comprising a source of carrier waves, a source of modulating signals, a pre-amplifier for said modulating signals, a power amplifier for the pro-amplified signals, a modulator tube, a modulation transformer coupling the plate circuit of said power amplifier to said modulator tube, a voltage divider bridged across the secondary winding of said modulation transformer, means to adjust the ratio of the sections of said voltage divider to correspond with a predetermined maximum peak voltage across said secondary winding, a voltage regulator tube of the diode rectifier type bridged across one section of said voltage divider, and circuit connections between said rectifier tube and said preamplifier tube to bias said pre-amplifier tube to 7 plate current cut-off when the modulator load is effectively removed from said secondary winding.

4. A modulation system according to claim 3, in which said voltage divider comprises a fixed resistor in series with an adjustable resistor across said secondary winding, said adjustable resistor being correlated with the desired per cent of modulation in the carrier to cause said rectifier tube to become conductive when the peak voltage developed across said secondary Winding tends to increase the per cent of modulation of the carrier above said predetermined level.

JOHN R. SHERWOOD. KENNETH L. KLIPPEL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,402,598 Charchian l June 25, 1946 FOREIGN PATENTS Number Country Date 560,906 Great Britain Apr. 26, 1944 

